Author Topic: T.C. measurements on precision resistors  (Read 281790 times)

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Offline branadic

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Re: T.C. measurements on precision resistors
« Reply #1100 on: January 22, 2020, 08:40:47 pm »
The resistor values measured can now be used to calculate a low t.c. 13:1 divider for e.g. LTZ temperature setpoint made out of 7 resistors as shown previously. Did that already and solved 8 possible equations as an example, though not all. And indeed I could find one combination resulting in a divider with very low temperature influence (blue curve), though power coefficient is not faced here and heat distribution within the network still has to be thought of, but I found it interesting to show anyway. The combination here is a series connection of R3+R4+R5 and parallel connection of R1||R6||R7||R8.

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« Last Edit: January 22, 2020, 09:02:38 pm by branadic »
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Online dietert1

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Re: T.C. measurements on precision resistors
« Reply #1101 on: January 22, 2020, 09:17:17 pm »
Last summer i used this little software to find the best combination. Hope it helps.

Regards, Dieter
 

Offline branadic

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Re: T.C. measurements on precision resistors
« Reply #1102 on: January 23, 2020, 05:18:50 pm »
Thanks dieter1 for the script, though I guess the t.c. alone is not enough, but you also need the consider absolute value of the individual resistor to optimize the network for low t.c., as they all have different absolute resistance and not perfect 5k. Thus I've used the resistor values instead of using their t.c. only for my manual "optimization".

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« Last Edit: January 23, 2020, 05:53:41 pm by branadic »
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Online Kleinstein

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Re: T.C. measurements on precision resistors
« Reply #1103 on: January 23, 2020, 06:00:00 pm »
The absolute values tend to be not that different. So for TC combinations one could use equal values.

Especially when using sets of 3 and 4 resistors, even picking a fixed combination of resistors should give a reasonable good match.
One also has to take into account that the TC may change after soldering to a board.
In an application like the LTZ1000 or a 7 to 10 V amplifier it is also not so much about the TC, but more about long time drift.
 

Offline branadic

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Re: T.C. measurements on precision resistors
« Reply #1104 on: January 23, 2020, 06:44:14 pm »
Kleinstein, I don't agree with you and the diagram above tell's another story. The slight difference in absolute value and t.c. leads to some residual t.c. once connected as a divider. However, noone knows how things change after soldering them to a board, I agree with you. And as for long term I would expect at least better behaviour compared to a divider made of single resistors. At least TDP networks have proven to work in W7000 and F7001 over long term.

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« Last Edit: January 23, 2020, 09:38:01 pm by branadic »
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Online dietert1

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Re: T.C. measurements on precision resistors
« Reply #1105 on: January 23, 2020, 07:13:40 pm »
It's a simple extension to combine TCs using measured resistor values, and the program won't run much longer. Let's see whether it makes a difference..

Regards, Dieter

PS: Meanwhile i checked and the approximation/simplification i proposed was a good one. I ran several examples of my Nomca data and it always found the best combination. The only noticeable change when taking into account measured resistor values is the optimum residual TC. It may be 10 ** -4 ppm/K off.
In order to avoid changes to the parts after characterization i used SMD solder adapters with gold plated pins in gold plated sockets (construction similar to HP 3456A thin film arrays). It's not only about soldering but also about cleaning, which should both be done before characterization.
« Last Edit: January 23, 2020, 09:32:04 pm by dietert1 »
 

Offline branadic

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Re: T.C. measurements on precision resistors
« Reply #1106 on: January 23, 2020, 09:03:15 pm »
I moved on and calculated the change in voltage at pin 6 of LTZ1000 over the temperature range of 10 ... 40°C due to the resistor network only, assuming a nominal zener voltage of 7.15V with the resistor arrangement previously found to perform good. It looks like a change of <3µV can be expected. Since this value is further attenuated by LTZ itself (0.01% --> 1ppm) we can call that zero. I think I need to setup a reference to prove it works in reality. But first I will measure another few samles to see, if one can find similar results within a batch of networks or if they have to be individually measured and connected.

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Offline RandallMcRee

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Re: T.C. measurements on precision resistors
« Reply #1107 on: January 23, 2020, 11:03:06 pm »
However, noone knows how things change after soldering them to a board, I agree with you.
-branadic-

What I do is to solder my precision resistors to some sort of carrier or just add wire-wrap leads. Only then measure.
 

Online Kleinstein

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Re: T.C. measurements on precision resistors
« Reply #1108 on: January 24, 2020, 08:29:55 am »
At least for the LTz1000 set point divider there is no need for a TC matching much better than some 5 ppm/K. The reference itself has some TC that for the non A version is adjusted with an extra resistor. If individually fitted this adjustment can also include a small TC from the set point.
So chances are good one can get away with a fixed PCB and use a measurement only as a check to see the final outcome. This could be after soldering the resistor array as one of the first parts.
 


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